Removing something and adding it back seems to be, by definition, not idempotent. What if something tries to access that file in between? What about the filesystem timestamps (same issues as with the "touch" claim)?
How do you guarantee the replacement symlink had the same chmod flags and grp flags as the original? (chmod -h for reference*)
I don't think these commands are idempotent in the wide. They satisfy only the narrowest "yea, most normal-ish cases are sort of maybe ok" but since we lost atomicity, there are significant windows of time in the filesystem where inodes are in flux, and the new thing has a different inode to the old thing, if you delete and replace it. The disk is different.
ln -sfn target new && mv -Tf new old